Computed tomogrophy scanning

ABSTRACT

Artefacts in the reconstructed volume data of cone beam CT systems can be removed by the application of respiration correlation techniques to the acquired projection images. To achieve this, the phase of the patients breathing is monitored while acquiring projection images continuously. On completion of the acquisition, projection images that have comparable breathing phases can be selected from the complete set, and these are used to reconstruct the volume data using similar techniques to those of conventional CT. Any phase can be selected and therefore the effect of breathing can be studied. It is also possible to use a feature in the projection image(s) such as the patient&#39;s diaphragm to determine the breathing phase. This feature in the projection images can be used to control delivery of therapeutic radiation dependent on the: patient&#39;s breathing cycle, to ensure that the tumour is in the correct position when the radiation is delivered.

FIELD OF THE INVENTION

[0001] The present invention relates to scanning by computed tomography(CT).

BACKGROUND ART

[0002] CT scanning is a process for imaging the internal structure of apatient. In conventional CT scanning, a beam of x-rays is projectedthrough the patient and its attenuation is measured. At the same time,the apparatus is rotated about an axis passing longitudinally throughthe patient. Thus, data is acquired as to the attenuation of the beam ineach direction in the plane in which rotation takes place. From thisdata, the internal structure of the patient on that plane can becomputed. The patient or apparatus is then indexed along the axis and afurther plane (known as a ‘slice’) is then investigated. A threedimensional image of the patient can then be constructed from thevarious slices.

[0003] One problem is that over the time required to acquire thenecessary slices, the patient is not motionless. Gross motor movementcan be avoided by suitable instruction to the patient, but even so eachslice is acquired at a different phase of the breathing cycle. Thisresults in a beating artefact due to the different frequency ofbreathing and slice acquisition.

[0004] Two ways have been used to solve this problem. On is to triggerthe CT on a particular phase of the patients breathing. This is termed‘respiration gated CT’ and implies that one CT slice is acquired forevery breath. This means that it takes a long time to acquire a completevolume of data.

[0005] Another technique is to monitor the phase of the patientsbreathing whilst acquiring CT slices continuously. Once the data isacquired, slices that have comparable breathing phase are selected fromthe complete set and these are then used to visualise the volume. Thishas the advantage that any phase can be selected retrospectively andtherefore the effect of breathing can be studied. This is termed‘respiration correlated CT’.

[0006] Conventional CT scans have the disadvantage that the resolutionalong the axis is poor since it corresponds to the slice thickness. Itis theoretically straightforward to increase this, but doing so resultsin a correspondingly longer acquisition time, or the need to rotate theapparatus correspondingly faster. Both options also give rise to anattendant reduction in contrast in the measured beam. Accordingly, ‘conebeam CT’ methods have been developed, in which a conical beam ofradiation is directed at the patient and a two-dimensional imageacquired via a flat panel detector. This apparatus is then rotatedaround the patient axis and a three-dimensional image is reconstructedfrom the set of two-dimensional images. As the individual slices areeliminated, there is the same resolution in all directions of the image.Likewise, as there are no slices the above-mentioned breathing artefactis absent since there can be no variation in patient position betweenslices.

SUMMARY OF THE INVENTION

[0007] We have however found that there are other artefacts in thereconstructed volume data of cone beam CT systems, which we have tracedto patient breathing movements. In addition, the motion is notmeasurable in the reconstructed volume data. This can be a particularproblem in cone beam systems due to the long time required foracquisition, typically 1-2 minutes.

[0008] The techniques used in conventional CT scanners cannot be useddirectly in a cone beam system as the data is acquired in 2D projectionimages, and therefore slices cannot be selected from the resulting data.However, respiration correlation techniques could be applied to theacquired projection images rather than the reconstructed CT volume. Toachieve this, we propose monitoring the phase of the patients breathingwhile acquiring projection images continuously. On completion of theacquisition, projection images that have comparable breathing phases canbe selected from the complete set, and these are used to reconstruct thevolume data using similar techniques to those of conventional CT. Anadvantage is that any phase can be selected and therefore the effect ofbreathing can be studied.

[0009] Breath control systems are available, intended for use inconventional CT scanning, and which could be used to monitor thepatient's breathing. As an alternative, however, it is possible to use afeature in the projection image(s) to determine the breathing phase. Asuitable feature is the position of the patient's diaphragm. This canthen be used to select the relevant images to be used in the projectionprocess.

[0010] It is known in the field of conventional CT scanning to beadvantageous to prompt the patient visually and audibly in order toensure a regular amplitude and pattern of breathing. Techniques such asthese could usefully be applied in the present invention.

[0011] Furthermore this feature in the projection images can be used tocontrol delivery of therapeutic radiation dependent on the patient'sbreathing cycle, to ensure that the tumour is in the correct positionwhen the radiation is delivered. This will provide a direct measure ofthe patient's breathing phase, a significant improvement as compared tocurrent methods that use external markers affixed to the patient. Theuse of a 3D volume data set generated using the same patient positionand contemporaneous with the treatment will remove significantuncertainties.

[0012] The present invention further provides a radiotherapy devicecomprising a respiration correlated cone beam CT scanner and a source oftherapeutic radiation, in which therapeutic radiation is deliveredduring the scan at times correlated with the patient's breathing cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] An embodiment of the present invention will now be described byway of example, with reference to the accompanying figures in which;

[0014]FIG. 1 is a view of a cone beam CT scanner according to thepresent invention, viewed along the axis of rotation thereof;

[0015]FIG. 2 is a schematic view of the system incorporating such ascanner; and

[0016]FIG. 3 shows a treatment apparatus including the scanner of thepresent invention.

DETAILED DESCRIPTION OF THE EMBODIMENT

[0017]FIG. 1 shows a cone beam CT scanner. A patient 10 is supported ona couch 12 which may be of any suitable design. Couches typically allowthe elevation and longitudinal position of the patient to be adjustedand this may be provided for as desired.

[0018] An x-ray source 14 is arranged to project a wide beam 16 ofradiation generally directed towards the isocentre 18 of the patient.The source 14 is rotatable around the isocentre 18 on a rotationalsupport 20. The support can; for example, be in the form of a ring orannulus around the patient 10 and couch 12 in which the source ismounted, or it can be a C-arm, or any suitable support allowing thesource to rotate, or any combination thereof.

[0019] A two-dimensional flat-panel detector 22 is also mounted on thesupport 20, opposite the source 14 and arranged to rotate in synchronismtherewith. If the support includes a C-arm then this can be achieved bymounting the detector on the opposite arm.

[0020] Thus, radiation emitted by the source 14 is partially absorbed bythe patient and the attenuated signal is detected by the flat paneldetector 22. The source 14 and detector 22 are then indexed rotationallyand a fresh image obtained. This is repeated until sufficient images areacquired to reconstruct the volume data, typically one completerotation.

[0021]FIG. 2 shows the system as a whole. The scanner of FIG. 1 isshown, together with cables linking the source 14, detector 22 androtational support 20 to a plurality of computing means 24, 26 whichprocess the data generated including the images, source intensity (etc),and rotational support position. Data is output via any suitable means,depicted generally as a monitor 28 but not limited thereto, and thesystem is controlled by any suitable input means, again depictedgenerally as a keyboard 30 but likewise not especially limited thereto.

[0022] As mentioned above, we have found that there are artefacts in thereconstructed volume data of cone beam CT systems, which we have tracedto patient breathing movements. To overcome or alleviate these,respiration correlation techniques are applied to the acquiredprojection images by the computing means 24, 26. This differs fromconventional respiration-correlated CT scanning in acting on theacquired projection images rather than the reconstructed CT volume.

[0023] To assist in this process, a breath control system is provided at32 to monitor the phase of the patients breathing while the projectionimages are acquired. On completion of the acquisition, projection imagesthat have comparable breathing phases can be selected from the completeset, and these are used to reconstruct the volume data using cone beamCT techniques. As a result, any phase or range of phases can be selectedand therefore the effect of breathing can be studied if desired.

[0024] As an alternative to the breath control system, it is possible touse a feature in the projection image(s) to determine the breathingphase, such as the position of the patient's diaphragm. This can then beused to select the relevant images to be used in the projection process.

[0025] An alert system including a light 34 and a buzzer 36 is provided,to prompt the patient visually and audibly in order to ensure a regularamplitude and pattern of breathing. Other alerts could of course beemployed, such as other forms of visible prompts including (for example)movable devices, and other forms of audible prompts including (forexample) speakers, percussive devices or any other form of controllablesound generation apparatus.

[0026]FIG. 3 shows a system including a therapeutic source of radiation38 arranged to emit a suitably collimated beam of therapeutic radiation40. This allows simultaneous scanning and treatment. If the radiationfrom source 14 continues during the treatment the selected feature(above) in the projection images can be used to control delivery oftherapeutic radiation from the source 38, dependent on the patient'sbreathing cycle. This ensures that the tumour is in the correct positionwhen the radiation is delivered. This will provide a direct measure ofthe patient's breathing phase, a significant improvement as compared tocurrent methods that use external markers affixed to the patient. Theuse of the same direct measure of the patient's breathing phase andpatient position to generate the 3D volume data set and control thetreatment delivery will remove significant uncertainties.

[0027] It will of course be understood that many variations may be madeto the above-described embodiment without departing from the scope ofthe present invention.

1. A method of cone beam CT scanning in which respiration correlationtechniques are applied to the acquired two-dimensional projectionimages.
 2. A method of cone beam CT scanning according to claim 1 inwhich the phase of the patient's breathing is monitored continuouslyduring acquisition of projection images.
 3. A method of cone beam CTscanning according to claim 2 in which projection images that havecomparable breathing phases are selected from the complete data set oncompletion of the acquisition and are used to reconstruct the volumedata.
 4. A method of cone beam CT scanning according to claim 2 in whicha feature in the projection image(s) is used to determine the breathingphase.
 5. A method of cone beam CT scanning according to claim 4 inwhich the feature is the position of the patient's diaphragm.
 6. Amethod of cone beam CT scanning according to claim 1 in which visualand/or audible prompts are provided for the patient's breathing.
 7. Amethod of cone beam CT scanning according to claim 1 in whichtherapeutic radiation is delivered during the scan at times correlatedwith the patient's breathing cycle.
 8. A cone beam CT scanner includingmeans for acquiring information as to the patient respiration cycle andmeans for selection of acquired two-dimensional projection images fromthe set of data acquired during a scan on the basis of the respirationcycle information.
 9. A cone beam CT scanner according to claim 8adapted to monitor the phase of the patient's breathing continuouslyduring acquisition of projection images.
 10. A cone beam CT scanneraccording to claim 9 arranged to select projection images that havecomparable breathing phases from the complete data set on completion ofthe acquisition and to use these to reconstruct the volume data.
 11. Acone beam CT scanner according to claim 9 including means for detectinga respiration-cycle-correlated feature in the projection image(s)thereby to determine the breathing phase.
 12. A cone beam CT scanneraccording to claim 11 in which the feature is the position of thepatient's diaphragm.
 13. A cone beam CT scanner according to claim 7including means to provide visual and/or audible prompts for thepatient's breathing.
 14. A radiotherapy device comprising a cone beam CTscanner and a source of therapeutic radiation, in which the CT scannerapplies respiration correlation techniques to the acquiredtwo-dimensional projection images and therapeutic radiation is deliveredduring the scan at times correlated with the patient's breathing cycle.